JPL/JSC Mars Sample Return Study II (1986)

Image: NASA

In 1983-1984, engineers and scientists at NASA’s Johnson Space Center (JSC), the Jet Propulsion Laboratory (JPL), and Science Applications, Inc. (SAI) performed a detailed Mars Sample Return (MSR) mission study. McDonnell Douglas Aerospace Corporation (MDAC) took SAI’s place on the team in the follow-on study that began in 1985.

The 1984 study and its sequel were very different in tone; the first was optimistic about an MSR mission, while its 1986 follow-on questioned the desirability of any further MSR planning. The former was shaped by President Ronald Reagan’s ringing January 1984 call for NASA to build an Earth-orbiting Space Station, the latter by the January 1986 Space Shuttle Challenger accident, which triggered a sweeping reassessment of the U.S. space program.

The 1984 study assumed that each MSR mission would need two Space Shuttle launches; one for the hefty MSR spacecraft and the other for a chemical-propellant Centaur G-prime upper stage that would launch the MSR spacecraft out of Earth orbit toward Mars. Centaur G-prime, a variant of the Centaur upper stage in use since the early 1960s, was designed specifically for launch in the Space Shuttle Orbiter’s 15-foot-wide, 60-foot-long payload bay.

At the time of the Challenger accident, Centaur G-prime’s maiden flight was scheduled for May 1986. Had the accident not intervened, the first Centaur G-prime would have reached Earth orbit attached to the Galileo Jupiter orbiter and atmosphere probe on board Atlantis, NASA’s newest Orbiter. After departing Atlantis‘s payload bay, the stage would have ignited to boost Galileo out of Earth orbit toward Jupiter (image at top of post).

The 1984 study’s MSR spacecraft and Centaur G-prime were to be brought together in orbit in either a Shuttle payload bay or a Space Station hangar. Spacecraft and upper stage would be launched separately because the 1984 MSR spacecraft would be too long and heavy for launch on board a Shuttle Orbiter with a Centaur G-prime attached.

The 1986 study emphasized size and mass reduction with the aim of launching the MSR spacecraft and its Centaur G-prime stage into Earth orbit on a single Shuttle. This had become the study’s focus, the team explained, because

the significance of being able to do the mission in a single shuttle launch has increased. The shuttle is much more expensive to launch than originally expected. . .Even for a large and relatively costly program such as Mars Sample Return, eliminating the expense of a second shuttle launch is significant. The relief to a tight launch schedule with a limited number of orbiters is significant as well.

Despite the JPL/JSC/MDAC team’s efforts to keep up with changing times, its work was rendered obsolete even as it was completed. Citing safety considerations in the aftermath of the Challenger accident, NASA cancelled Centaur G-prime in June 1986, a month before the JPL/JSC/MDAC team’s MSR study report saw print. This left NASA planetary missions designed for Shuttle-Centaur G-prime launch with no means of reaching their destinations. Solid-propellant upper stages, planetary gravity assists, and expendable launch vehicles would subsequently replace the Shuttle-Centaur G-prime system in NASA’s planetary mission plans.

Obsolescence should not, however, be confused with irrelevance. The 1986 study remains important as a step in the evolution of MSR planning in the 1980s, and it is illustrative of the forces shaping robotic planetary exploration in the same period.

The 1984 MSR study had looked at eight mission design options before arriving at a baseline. The 1986 study arrived at four possible baseline mission designs, three of which showed promise for enabling the MSR spacecraft and Centaur G-prime to launch together on a single Space Shuttle.

The 1986 study’s first plan, designated Option A1, was very similar to the 1984 study’s baseline option. A two-part “bent biconic” aeroshell would protect the MSR spacecraft during aerocapture, when the spacecraft skim through Mars’s atmosphere to slow down so that the planet’s gravity could capture it into Mars orbit.

After aerocapture, the aeroshell aft section containing the Mars Orbiter Vehicle (MOV) and Earth Return Vehicle (ERV) would separate. The forward section (the Mars Entry Capsule, or MEC) would fire a rocket to slow down and drop into the atmosphere a second time so that it could aeromaneuver to its landing site. As it neared the site, the Mars Lander Module (MLM) would deploy a parachute and separate from the aeroshell, then would ignite rockets to descend to a soft landing.

The 1986 study team’s Option A1 MSR spacecraft had an estimated mass to 8118 kilograms, or 1375 kilograms less than the 1984 baseline spacecraft. A Shuttle carrying a fully fueled Centaur G-prime could tote an additional 7800 kilograms into Earth orbit. The JPL/JSC/MDAC team admitted that Option A1 was “still somewhat too heavy for a single [Shuttle] launch,” and added that, unless “there are substantial technical breakthroughs, it is difficult to see how the mass can be reduced enough to bring it within the single launch range.”

The team pointed out, however, that, unlike its 1984 counterpart, the Option A1 MSR spacecraft could fit into a Shuttle payload bay while attached to a Centaur G-prime. Furthermore, spacecraft and stage could reach orbit on board a single Shuttle if the latter were launched with a partial propellant load and “topped off” in orbit at the Space Station or by scavenging liquid oxygen/liquid hydrogen propellants left over in the Shuttle’s External Tank (ET). The latter option assumed that the Shuttle Orbiter would carry the ET into orbit; this would, however, represent a new capability, since normally the ET would be cast off just short of achieving orbital velocity. It also assumed that NASA would develop equipment for scavenging leftover ET propellants.

The 1985 JSC/JPL/MDAC team’s option B1. The MEC and MOV would leave Earth orbit and cruise to Mars in separate aeroshells, then would go their separate ways at Mars. DE = Direct Entry. Image: NASA

The JPL/JSC/MDAC team’s second option, labelled Option B1, included the only MSR spacecraft light enough (7008 kilograms) to reach Earth orbit on board a Shuttle Orbiter attached to a fully fueled Centaur G-prime stage. The spacecraft would comprise two parts, each packed within a separate bent biconic aeroshell. The smaller aeroshell would carry the MOV and ERV, while the larger would contain the MEC.

Upon arrival at Mars, the two aeroshells would separate. The MEC would dive directly into the martian atmosphere, aeromaneuver to its landing site, cast off its aeroshell, and land. The MOV/ERV, meanwhile, would perform aerocapture into Mars orbit. The team noted that packaging the two aeroshells to fit together inside the Shuttle Payload Bay and attaching them to the Centaur G-prime would demand a complex and heavy support structure. Because of this, Option B1, though “promising on paper,” had to be “viewed with some suspicion in terms of both volume and mass.”

Option A2 would see the MSR spacecraft perform a propulsive Mars Orbit Insertion maneuver; because of this, the MOV/ERV would not require an aeroshell. The MEC would enter Mars’s atmosphere only to aeromaneuver and land. Image: NASA

Option A2 was similar to the mission plan the twin Viking spacecraft followed in 1976. The MSR spacecraft would ignite a rocket engine to slow down so Mars’s gravity could capture it into orbit, then the MEC lander would separate from the MOV/ERV and fire a rocket to descend into the atmosphere, where, unlike the Vikings, it would aeromaneuver to reach its landing site.

At 12,537 kilograms, the Option A2 MSR spacecraft was “by far the most massive of the lot.” With an attached fully fueled Centaur G-prime, it would far exceed the launch capability of a single Shuttle Orbiter. It would, the team reported, be “marginal” even if the attached Centaur G-prime were launched empty and fueled in Earth orbit.

The team’s fourth and final option, designated B2, would be similar to the mission plan the Soviet Mars 2 and Mars 3 probes used for their failed landing missions in 1971. The MEC would separate from the MOV/ERV during final approach to Mars and enter the atmosphere directly. As in the other options, it would aeromaneuver to its landing site in a biconic aeroshell. The MOV/ERV, meanwhile, would fire a rocket and enter Mars orbit. The team judged that this concept, though heavier (8672 kilograms) than either Option A1 or B1, could “become very desirable because of the flexibility it allows.”

The amount of propellant needed to place the Option B2 MOV/ERV into low circular Mars orbit might, for example, be dramatically reduced through aerobraking. In that scenario, the MOV/ERV would fire a rocket motor to slow down only enough so that Mars’s gravity would capture it into a loosely bound elliptical orbit. It would then skim through the planet’s upper atmosphere repeatedly over a period of weeks to lower and circularize its orbit.

In recent years, Mars orbiters have employed this technique to reach their final mapping orbits; Mars Global Surveyor (MGS), which arrived in Mars orbit in September 1997, was the first. After a delay caused by a damaged solar array that threatened to buckle under the strain of aerobraking, MGS reached its mapping orbit in April 1999.

The EAC bearing the Mars sample would ride in the ERV from Mars to Earth. Image: NASA

The JPL/JSC/MDAC team added to all four of its MSR mission options its chief mass-saving technique: aerocapture at Earth. A 2.2-meter-long, 0.9-meter-wide biconic Earth Aerocapture Capsule (EAC) would replace the 1984 study’s propulsively braked Earth Orbit Capsule.

The EAC would travel from Mars orbit to Earth’s vicinity inside a drum-shaped, 3.15-meter-long, one-meter-wide ERV with two solar panel “wings.” It would separate from the ERV and skim through Earth’s upper atmosphere at a height of about 70 kilometers to slow down.

After leaving the atmosphere, it would discard its aeroshell to expose a solid rocket motor and solar cells (the latter would power a radio beacon that would aid recovery). When the EAC reached apoapsis (the high point in its orbit), it would fire its rocket to raise its periapsis (the low point of its orbit) above the atmosphere. In addition to saving propellant (hence mass), Earth aerocapture would place the Mars sample in a low circular orbit within reach of an Orbital Maneuvering Vehicle (OMV) remote-controlled from a Shuttle Orbiter or the Space Station.

The JPL/JSC/MDAC team then described other mass-saving modifications to the 1984 MSR plan. First, it reduced the mass of the Sample Canister Assembly (SCA) by reducing the size and number of sample vials it could carry. The new SCA would pack 19 234-millimeter-long, 30-millimeter-diameter vials into a drum 0.4 meters in diameter and 0.5 meters long. The narrower, lighter SCA would mean that the 1986 Mars Rendezvous Vehicle (MRV) that would launch it into Mars orbit could be made smaller than its 1984 counterpart (4.8 meters long by 1.8 meters in diameter versus 5.37 meters by 1.84 meters).

In a further departure from the 1984 study, the 1986 study’s sample-collecting rover would not carry the SCA; it would instead return to the MRV each time it filled a sample vial and transfer it to the SCA located there. The JPL/JSC/MDAC study team opted for this approach to help to ensure that at least a partial sample could reach Earth in the event of a rover failure before the SCA was filled.

Upon arriving back at the lander, the rover would use its robot arm to place individual filled sample vials inside the SCA in the MRV. A robot arm on the MLM would provide redundancy; it would be capable of transferring the vials to the SCA if the rover’s arm malfunctioned, or it could collect a “grab” sample from close by the MLM if the rover failed to collect any samples.

The Mars Lander Module uses its robot arm to transfer the Sample Canister Assembly from the sample-collection rover to the Mars Rendezvous Vehicle. Image: NASA

Unlike the 1984 MRV, which soon after arriving on Mars would pivot to point its dome-shaped nose at the sky, the 1986 MRV would remain horizontal until just before planned launch. This would enable the rover to load samples directly into the SCA in the recumbent MRV’s nose, eliminating the need for the 1984 MLM’s crane-like SCA Transfer Device. Because the 1986 MRV would be smaller, the MLM could also be smaller. This would permit a shorter, less massive MEC (8.1 meters long versus 12.2 meters in the 1984 design). The team also added a fourth landing leg to improve MLM stability.

The 1986 team retained the Mars Orbit Rendezvous scheme of the 1984 study. The MRV would blast the SCA to Mars orbit, then the MOV/ERV would rendezvous and dock with MRV. The MRV would transfer the SCA automatically to the EAC within the ERV, then the MOV/ERV would cast off the MRV.

The 1986 MOV would, the team reported, have an “unconventional” design. A compact assemblage of propellant and pressurant tanks affixed to a rectangular box would replace the 1984 MOV’s tidy hexagonal drum. This would reduce the MOV’s length from 4.5 meters to 2.8 meters. The ERV, with four solid-rocket motors for Mars orbit departure, would nest inside the box, further limiting length. Together these steps would contribute toward an MSR spacecraft design short enough to fit inside the Shuttle Orbiter Payload Bay while attached to a Centaur G-prime.

The JPL/JSC/MDAC team concluded its report by proposing possible follow-on study areas. Before it did, however, it noted that Mars mission planning was “somewhat uncertain at the moment” because of the National Commission on Space (NCOS) planning effort. The NCOS exercise, led by former NASA Administrator Thomas Paine, was a congressionally mandated Reagan Administration effort aimed at giving NASA long-term goals. Pending completion of the NCOS report and “the official reaction” to its recommendations, the team wrote that

it seems of little utility to indulge in yet another year of system studies of the Mars Sample Return mission, a subject that has already been most thoroughly studied. Until a strategy for Mars exploration becomes clear, such studies. . .may not be particularly useful. If the nation chooses to pursue. . .an early manned mission. . .there is little reason and, probably, inadequate time to carry out an unmanned sample return first. On the other hand, if a more deliberate pace is chosen, which pushes a manned [Mars] mission past the first decade of the next century, then the [MSR] mission is much more attractive. . .

Mindful of this uncertainty, the team proposed that JPL work with JSC on strategies and technologies “supportive of both manned and unmanned Mars exploration.” and that JSC study piloted Mars missions and Mars sample collection and handling. It wrote that JPL study areas might include manufacture of propellants on Mars from resources found there, aerocapture/aeromaneuver analysis, laser ranging for Mars Orbit Rendezvous maneuvers, and rover guidance and navigation on Mars’s surface. The team cautioned, however, that these technology development activities would depend “upon a resolution of funding issues.”

Six months after the JPL/JSC/MDAC MSR study report saw print, the NASA-sponsored Mars Study Team (MST) completed a report calling for an international Mars Rover Sample Return (MRSR) mission. The MST, which included many scientists who had participated in the 1984-1986 MSR studies, envisioned that the U.S. would contribute the mission’s sophisticated rover. Six months after that, the high-profile Ride Report threw a bright spotlight on MRSR. Though funding issues remained, the MRSR concept moved to the center of NASA planning for robotic Mars missions.